Check for space membership during a remote join of a restricted room (#9814)

When receiving a /send_join request for a room with join rules set to 'restricted',
check if the user is a member of the spaces defined in the 'allow' key of the join rules.

This only applies to an experimental room version, as defined in MSC3083.

synapse/api/auth.py

@@ -65,6 +65,7 @@ class Auth:
     """
     FIXME: This class contains a mix of functions for authenticating users
     of our client-server API and authenticating events added to room graphs.
+    The latter should be moved to synapse.handlers.event_auth.EventAuthHandler.
     """
 
     def __init__(self, hs):